It is well known that carbon fibers having excellent properties suitable for commercial use can be produced from mesophase pitch. Mesophase pitch derived carbon fibers are light weight, strong, stiff, thermally and electrically conductive, and both chemically and thermally inert. The mesophase-derived carbon fibers perform well as reinforcements in composites, and have found use in aerospace applications and quality sporting equipment.
Low cost carbon fibers produced from isotropic pitch exhibit little molecular orientation and relatively poor mechanical properties. In contrast, carbon fibers produced from mesophase pitch exhibit highly preferred molecular orientation and relatively excellent mechanical properties.
The term "pitch" as used herein means petroleum pitches, natural asphalt and heavy oil obtained as a by-product in the naphtha cracking industry, pitches of high carbon content obtained from petroleum asphalt and other substances having properties of pitches produced as by-products in various industrial production processes.
The term "petroleum pitch" refers to the residuum carbonaceous material obtained from the thermal and catalytic cracking of petroleum distillates or residues.
The term "anisotropic pitch" or "mesophase pitch" means pitch comprising molecules having aromatic structure which through interaction have associated together to form optically ordered liquid crystals.
The term "isotropic pitch" means pitch comprising molecules which are not aligned in optically ordered liquid crystals.
The term "mesogens" means mesophase-forming materials or mesophase precursors.
Mesophase pitch is not ordinarily available in existing hydrocarbon fractions, such as refining fractions, or in coal fractions, such as coal tars. Mesophase pitch, however, may be derived from isotropic pitch containing mesogens. Isotropic pitch containing mesogens is usually prepared by the treatment of aromatic feedstocks. Such treatment, which is well known in the art, may involve one or more heat soaking steps, with or without agitation, and with or without gas sparging or purging. Gas sparging may be carried out with an inert gas or with an oxidative gas, or with both types of operations. Numerous patents describe various aspects of the treatment of aromatic containing feedstocks to obtain isotropic pitch. Included are: U.S. Pat. Nos. 4,209,500, heat soaking; 3,976,729 and 4,017,327, agitation during heat treatment; 3,974,264 and 4,026,788, inert sparge gas during heat treatment; 4,283,269, heat soaking of fluxed pitch; Japanese Patent No. 65090/85, heating in the presence of an oxidizing gas; U.S. Pat. Nos. 4,464,248, catalytic heat soaking; 3,595,946 and 4,066,737, use of oxidative reactive material; and 4,474,617, use of oxidizing gas; and many others.
Mesophase pitch may be obtained from isotropic pitch containing mesogens by solvent fractionation, which is carried out by the following steps:
(1) Fluxing the isotropic pitch in a hot solvent. PA1 (2) Separating flux insolubles by filtration, centrifugation, or other suitable means. PA1 (3) Diluting the flux filtrate with an anti-solvent (comix solvent) to precipitate a mesophase-forming (mesogen containing) pitch. PA1 (4) Washing and drying the precipitated pitch. PA1 (5) Fusing the precipitated pitch to form mesophase.
The solvent fractionation procedure described is well known in the art and is set forth in some detail in numerous patents, including U.S. Pat. No. 4,277,324, which is incorporated herein by reference. This patent sets forth numerous solvent and anti-solvents which can be employed in solvent fractionation and the operating conditions and procedures which may be used.
Separation of mesogens from isotropic pitch may also be effected by the solvent extraction process described in U.S. Pat. No. 4,208,267. In this patent fractionation is accomplished without fluxing or flux filtration. The mesogen-containing isotropic pitch is extracted with a comix type solvent and the mesogens are collected as an insoluble residue. Solvents used in this process are similar to those employed in the process of U.S. Pat. No. 4,277,324.
It is desirable to provide an alternative process for obtaining mesophase pitch from isotropic pitch which does not involve the use of a comix solvent, and thus eliminates the need for storage and pumping facilities for two solvents and separation facilities for separating the solvents.
According to the present invention, isotropic pitch containing mesogens is combined with a solvent and subjected to dense phase or supercritical conditions to effect phase separation of the mesogens from the pitch. In one aspect of the invention, isotropic pitch containing mesogens is fluxed with a solvent to solubilize the mesogens, the flux mixture is then filtered to remove insolubles, and the solubilized mesogens are phase separated from the flux mixture under dense phase or supercritical conditions of temperature and pressure. The dense phase or supercritical conditions employed are such that the mesogens are recovered as mesophase.